Q&A: A Clear Finish For Wood Siding

Cutting Concrete

A.Byron Papa
responds: I’ve found that almost no
clear finish holds up more than a year outside.
Clear coatings allow the ultraviolet (UV) rays in
sunlight to penetrate, degrading the outermost
layer of cells in the wood siding. Add water from
rain and humidity (we get plenty of both here in
North Carolina) and tiny mildew spores begin
growing on the surface of the wood. This further
breaks down the bond between the wood and finish.
Consequently, with almost any clear finish, you
have to recoat each year.

The only thing I’ve found that holds up
better is an oil-based clear coating from Sikkens
(AKZO Nobel Coating, P.O. Box 5062, Troy, MI 48007;
800/833-7288). This goes on like a varnish, but the
surface buildup is 30 to 35 microns thick, which is
two to three times thicker than most other
coatings. (For the West Coast, where legislation
limits the use of high-VOC products, Sikkens is
working on another product. Most likely this will
be oil-based with a high solids content.)

The Sikkens coatings aren’t absolutely
clear. They have a synthetic iron-oxide pigment.
Though they have a variety of tints, I’ve
used only a natural cedar color, which turns cedar
the reddish brown of an unfinished mahogany. The
iron oxide in the finish actually reflects UV
light, so it can’t penetrate the coating
and break down the underlying wood as fast. Sikkens
claims to grind the iron-oxide particles much finer
than other coating manufacturers. Because the
finish is so thick and the particles so fine, you
get a nearly impenetrable coating of iron oxide. I
get about three years out of the product before a
house needs another maintenance coat.

On the down side, the Sikkens coatings are
expensive — on the order of $45 per
gallon. If you’re spending that kind of
money, it’s only worth putting them on a
premium wood siding, such as redwood, western red
cedar, or cypress. And it’s worth taking
the time to apply them right the first time. This
includes good prep.

Before applying the finish, power wash new
siding, either with an oxalic-acid cleaner, such as
Dekswood (The Flood Co., P.O. Box 399, Hudson, OH
44236; 800/321-3444), or with trisodium phosphate.
This raises the grain and allows the finish to
penetrate better. While any clear finish will stick
better to rough-sawn siding, it’s
absolutely essential to wash smooth siding, which
develops a "mill glazing" that closes the pores in
the wood.

Let the wood dry. Here in the Southeast, this
can take six to eight weeks. But this step is
probably as important as washing. The weathering
helps open the pores in the wood surface to allow
the finish to penetrate.

I apply three coats with a white China bristle
brush (it pays to lay out money for a good brush).
For the first coat I use Sikkens’ Cetol 1,
which is thin, like a primer, and penetrates well
into the wood. For coats two and three, use the
Cetol 2-3 Plus, which is thicker. For decks and
other horizontal surfaces, Sikkens makes DEK Base
and Cetol DEK, which have superfine silica
particles to help resist the abrasion of foot
traffic.

Byron Papa is a painting and remodeling
contractor in Chapel Hill, N.C.